This invention pertains to a process for enhancing the melt strength of a polypropylene (co)polymer by mixing an initiator with the polypropylene (co)polymer at a temperature below the decomposition temperature and heating the mixture, with most of the initiator decomposing before the polymer has melted and with the formed radicals reacting with the polymer.
Such a process is known from DE-A-4,340,194, where a polypropylene homopolymer or copolymer is mixed with bis(2-ethylhexyl) peroxydicarbonate, after which, as a result of the initiator decomposing and an after treatment with heat in an inert atmosphere, a modified polymer with enhanced melt strength is obtained. For greater ease of handling and effective distribution through the polymer, the initiator employed is diluted with an inert solvent. However, there are several drawbacks to the process disclosed in the German patent application. In particular, the handling of the bis(2-ethylhexyl) peroxydicarbonate that is utilized is attended with certain risks, among them premature (explosive) decomposition. It is true that this risk is reduced by mixing the initiator with an inert solvent, but this means that a substantial quantity of an undesired additive is introduced into the end product. Besides, the required storage temperature and the temperature at which it is still possible to handle the dissolved initiator continue to be low. The patent publication also shows that only one specific initiator, the aforesaid bis(2-ethylhexyl) peroxydicarbonate, is suitable for increasing the melt strength of the polymer.